It is now well known that olives contain a number of bioactive compounds, particularly polyphenols; among these polyphenols, hydroxytyrosol is of outstanding biological significance in view of its antioxidant, antimicrobial and radical scavenging activity.
Hydroxytyrosol has the following formula:

Production of hydroxytyrosol from olives residues after oil extraction was and is actively investigated; an efficient extraction process could be very profitable especially because the major amount of hydroxytyrosol and hydroxytyrosol precursors initially present in the olives remains in the residues from olive oil production and only a minor part is found in the oil. Extra virgin olive oil normally contains 1-20 ppm of hydroxytyrosol.
The residues of olive oil extraction of interest for the present application exclude the olive tree leaves, because the leaves are removed before oil extraction. Moreover, hydroxytyrosol extraction from leaves faces different starting compositions (and therefore different extraction problems) than extraction from olives or olive residues.
The residues of olives as obtained from olive oil extraction processes of interest for the present application can be classified as:                pomaces, i.e. the solids containing residues of the pressing (in spanish: orujo), of the three-phases process (orujo), or of the two-phases process, in which no water is added to the chopped olives in the centrifugation step (in spanish: alperujo). Orujo, alperujo and defatted orujo contain a high amount of water (45% to 70%). Extraction residues also comprise orujillo, the olive dry solids, after orujo oil extraction that contains less than 15% water and is nearly free from oil residues.        
The green olives extracts of the invention are preferred to the extracts of residues of olive oil production in view of their greater amount of hydroxytyrosol and of the reduced content of hydroxymethylfurfural.
For the purposes of the present description the term “pomaces” or “solid residue” is designating both “orujo”, “defatted orujo” “orujillo” and “alperujo”. Preferred starting materials for the present invention are olives, more preferably green olives, and pomaces.
It is known to carry out acid hydrolysis of the pomaces (or of vegetation water) to have the cleavage of the ester bond in the oleuropein molecule and obtain hydroxytyrosol.
U.S. Pat. No. 6,361,803 discloses extraction of hydroxytyrosol (and other compounds) by neutral or acid hydrolysis of olive pulp residues at reflux for one hour (ex. 12). The extracted water solution is loaded on an absorption XAD-7 column that is eluted with methanol to recover the extracted hydroxytyrosol. U.S. Pat. No. 6,361,803 requires the use of organic polar solvents, to recover hydroxytyrosol with a minimum purity grade, in addition (ex. 12) freeze precipitation of some impurities from the methanol solution is necessary. Polar aqueous solvents are selected among methanol, ethanol, acetonitrile or acetone, while polar organic solvents are selected, for example, among esters, amides, dimethyl sulfoxide, dioxane, DMF and their mixtures. Most of these solvents are toxic and very difficult to completely eliminate from the desired hydroxytyrosol product. Accordingly, traces of the utilized solvents will be found in the final product even after several purification steps, thus rendering the hydroxytyrosol obtained according to this process not suitable for a safe application in the alimentary, cosmetic and pharmaceutical field. Moreover, the final product is not suitable for use in fortifying foods, particularly edible oils, without use of ethanol and acetic acid as additives to the extract to obtain a stable food product: the resulting food product (oil plus hydroxytyrosol containing extract, ethanol and acetic acid) is not acceptable in the food industry.
WO 2004/005228 discloses hydrolysis at room temperature of vegetation water obtained from olive oil extraction by incubation of the acidified vegetation water for at least two months and preferably 6-12 months until at least the 50% (preferably 90%) of the oleuropein originally present in the vegetation water has been converted to hydroxytyrosol. The incubated vegetation water is extracted with an organic solvent, for example ethyl acetate, or it is contacted with a supercritical fluid (CO2), to produce a rich fraction in hydroxytyrosol. The main problems of this process are the very long time required for the incubation of the acidified vegetation water and the use of organic solvents, that should be avoided, particularly when the final product obtained is to be used in the alimentary, cosmetic and pharmaceutical field.
US 2004/0102657 discloses acidic hydrolysis with a steam explosion process at high temperatures (about 190-220° C.). The obtained solution is firstly partially purified on a column with non activated ion exchange resin and subsequently loaded on a XAD non-ionic column, from where the hydroxytyrosol is eluted with methanol or ethanol. The process results in a poor yield in hydroxytyrosol, considering the process in its entirety.
EP 1623960 discloses a process of recovery of hydroxytyrosol, and tyrosol, from alpechin by means of filtration in a complicated plant consisting of three units (Ex. 1) and subsequent separation. The tyrosol is then oxidated to hydroxytyrosol in a protic solvent (alcohol or water) to obtain a final product that is semi-synthetic. Additionally, EP 1623960 discloses that concentration by nanofiltration and reverse osmosis of vegetation waters (alpechin) are carried out at neutral or alkaline pH (claim 4). This process has two important problems. A first drawback is that hydroxytyrosol degradation increases at neutral and alkaline conditions appearing unwanted products that are difficult to remove, and, in addition, due to the fact that no acid hydrolysis is carried out the oleuropein content should be still high. Secondly, the obtainable concentration factor is poor, thus the maximum concentration allowed for hydroxytyrosol according Ex-1 (see table 1) is in the range from 1.2 to 1.6 g/L.
US 2004/0176647 discloses an extraction process of phenols from alperujo under stirring at 180-240° C. in autoclave, in water. No acid is added, but the thermal treatment results in a “liberation of acetyl groups” and in an alleged consequent reduction of the pH (page 3, first paragraph). Nevertheless, at the pHs produced in the described conditions hydrolysis of oleuropein is far to be complete, meaning two tings, lower hydroxytyrosol yields and remaining hydrolysed oleuropein, that as we discuss above is associated causing non-pleasant taste in food products containing such extracts, being then not suitable for food applications. The products are tyrosol and hydroxytyrosol, that are separated by HPLC with sulphuric acid/acetonitrile eluent.
Summarizing, the above mentioned techniques are either too long or too complex, or too harsh, or all of the above; this results in that the amount of remaining oleuropein, i.e. the amount that is not hydrolyzed, and/or the amount of the hydrolysis by-products such as hydroxymethylfurfural is high enough to make difficult the subsequent purification steps.